Our analysis demonstrates that G 9-40 b is inconsistent with being a bare rock and that its composition may range between a water world with a steam atmosphere and a mostly rocky planet with a large hydrogen-rich envelope. Therefore, its location is consistent with the predictions from gas-poor formation models [1]}, [2]}, [3]}, but inconsistent with those from atmospheric-mass loss. While G 9-40 b, TOI-1685 b, K2-146 b, and TOI-776 b are all consistent with this scenario, the terrestrial planets TOI-1634 b and TOI-1235 b contradict it. Furthermore, we do not find in our sample the apparent transition between theories as a function of stellar mass suggested by [4]} and [5]}. As an example, the ultra-short period planets TOI-1634 b and TOI-1685 b both orbit very similar hosts (\(M_\star \sim 0.5\,M_\odot \) ) despite their differences in internal composition. Enlarging the sample of precisely characterized keystone planets with additional information on the age of the system [6]}, [7]}, [8]} and reliable stellar abundances [9]}, [10]} is a way forward towards identifying the dominant mechanism sculpting the radius distribution of small planets orbiting M dwarfs, and to look for differences (if any) with solar-type stars.
